The present invention generally relates to electronic digital controls for gas turbine engines and, more particularly, to a system and method for generating consolidated gas turbine control tables for engine power setting.
Known electronic digital controls for gas turbine engines contain tables for engine power setting. As disclosed in U.S. Pat. No. 6,311,106 to Dupont, an embedded table in the engine control is used as part of the engine power setting adjustment for aircraft payload. From a total gross weight of the aircraft including the weight of loaded cargo containers, a required flat rate engine power for the aircraft is determined by inputting the total gross weight into a look-up table. The required flat rate engine power is sent as a command signal to the engine control system.
For aircraft engines, specifically turbofans, an engine power setting parameter (fan speed for example) may be scheduled as a function of variables such as altitude, flight speed and ambient temperature. The engine control interrogates the power setting table at a specific flight condition of interest and determines a power setting parameter set point. Engine fuel flow may then be modulated so that a measured engine parameter agrees with the set point value.
To minimize the power setting error for interpolated conditions, conventional power setting tables are densely populated. The tables typically cover all environmental conditions regardless of their probability of occurrence. For example, an engine power set point is required at sea level and static conditions. However, an engine power set point is not required at high altitudes, 50,000 feet for example, and static conditions as this condition is outside normal engine and aircraft flight envelopes. Nevertheless conventional power setting tables include an entry at high altitude and static conditions. Furthermore, typical power setting tables include large portions that contain data for flight conditions that may never be encountered.
Generating tabular power setting data for regions that are outside the normal engine operating envelope is particularly arduous, prone to error and impossible to verify. With reference to FIG. 1 and FIG. 2, there are shown operating envelopes generally designated 100 and 200 correlating flight speed and altitude and ambient temperature and altitude respectively. Data locations 110 and 210 show conditions where engine power setting parameters are generated. As can be seen, a substantial portion of the generated power setting parameters lie outside operating envelopes 100 and 200.
For each value of flight speed and ambient temperature used to generate a power setting parameter a corresponding location 110 and 210 exists for each altitude. Therefore, in order to provide adequate data density or fidelity at any one altitude, all other altitudes require defined data regardless of need which leads to large tables and memory requirements. However, memory allotted to engine controls is limited by considerations of power supply requirements, storage size, heat generation and cost. Consequently, tables such as those represented in FIG. 1 and FIG. 2 must be small and have reduced fidelity. The reduced fidelity can lead to undesirable thrust scheduling such as where the engine delivers slightly more thrust than required and consequently operates at elevated turbine temperatures. In turn, elevated turbine temperatures must be considered as part of engine design.
Data locations 110 and 210 located outside of operating envelopes 100 and 200 respectively can be extremely difficult to generate. Many of such data locations 110 and 210 can be beyond the tested and/or analytical predictions of the components that comprise the engine model. Engine models typically used in these regions often do not converge so that other extrapolation methods are used to generate the necessary data points. Typically, the extrapolation methods are purely mathematical and do not necessarily adhere to generally accepted laws of physics. Consequently, the extrapolated data has to be checked thoroughly to ensure that it does not introduce any anomalies near the operating envelopes 100 and 200.
With reference to FIG. 3 and FIG. 4, data locations 310 and 410 exist only within operating envelopes generally designated 300 and 400 respectively. A comparison of the number of data locations 310 to the number of data locations 110 and a comparison of the number of data locations 410 to the number of data locations 210 shows nearly a 60% reduction in tabulated data. However, the tables represented in FIG. 3 and FIG. 4 do not satisfy the engine control table lookup requirement of a flight speed and ambient temperature value for each altitude. This requirement is a result of the use of conventional table interpolation routines to generate power setting parameters.
A typical process for power setting parameter lookup is shown in FIG. 5. Values of altitude, flight speed and temperature are input to a process 500 which interrogates a table and generates a power setting parameter corresponding to the inputs.
As can be seen, there is a need for a system and method for generating consolidated gas turbine control tables that significantly reduces the amount of data required by the control table. Such a system and method preferably provides fidelity in the normal engine and aircraft operating envelopes. Further, such a system and method preferably only generates data required in the normal operating envelopes and thereby results in a reduced table size. Such a system and method also preferably provides for increased power setting parameter fidelity only where it is needed without unduly affecting operation of the engine far from the normal operating envelopes.
In one aspect of the present invention, a method for generating a power setting parameter table includes the steps of generating a non-dimensional index from a plurality of first and second inputs and determining a power setting parameter corresponding to each index and second input. The first inputs may include flight speed and temperature values and the second inputs may include altitude values.
In another aspect of the present invention, a system for generating a power setting parameter table includes a memory coupled to a processor, the processor operable to generate an index from a plurality of first and second inputs and determine a power setting parameter corresponding to each index and second input.
In yet another aspect of the present invention, a computer readable media for generating a power setting parameter table includes a code segment for generating an index from a plurality of first and second inputs and a code segment for determining a power setting parameter corresponding to each index and second input.
In a further aspect of the present invention, a method for generating a power setting parameter includes the steps of generating a first index from a plurality of first and second inputs, determining a first power setting parameter corresponding to each first index and second input to form a power setting parameter table, and interrogating the power setting table using the first index and the second input to generate the first power setting parameter.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.